This invention relates to a method for checking the condition of containers.
In particular, the invention relates to a method for checking the condition of plastic containers containing concentrated products such as, for example, liquids having a high density coefficient, products in the semi-liquid or semi-solid state and the like.
The invention also relates to a test head and a machine for checking the condition of containers of said kind.
As is known, containers of this kind are filled with the product they are designed to contain and then hermetically sealed by applying a membrane.
During packaging and, in particular, during sealing operations, the container may be incorrectly sealed or the sealing membrane may have imperfections in it such as small- or medium sized holes, not detectable by the human eye, or even large-sized holes.
In such cases, the container must be scrapped since the product inside it is not hermetically sealed or adequately protected.
Machines for checking the condition of containers after packaging are known in the prior art. These machines comprise a plurality of bell-shaped test heads designed to isolate the container hermetically and to create a negative pressure (or vacuum) around the container for a defined interval of time.
During this interval of time, when the container is subjected to the negative pressure, the test heads detect whether or not the membrane expands by more than a defined, preset threshold value.
Exceeding the threshold indicates that the container is intact because negative pressure applied to an undamaged container creates a vacuum action that deforms the membrane and causes it to swell.
If the membrane does not swell or the swelling does not exceed the threshold, then the machine tells the operator that the container is not intact.
That is because a hole or sealing defect in the membrane causes the air inside the container to escape to the outside when the negative pressure is applied by the test head to create the vacuum action around the container.
The air extracted from the container compensates the vacuum action of the test head and the membrane either does not swell at all or, at most, swells by an amount less than the threshold.
In that case, the machine indicates that the container does not conform with specifications.
Prior art machines of the above type have some drawbacks.
Especially in the case of containers containing a highly viscous product, a part of the viscous product might find its way into a hole in the membrane as a result, for example, of the container being tilted over during the packaging process.
In that case, the negative pressure created by the test bell does not extract the air from the container because the hole is obstructed by the semi-solid (or semi-fluid) product.
The vacuum action of the test head is not sufficient to remove the product from the hole and the membrane swelling exceeds the set threshold as if the membrane were intact.
Thus, the machine does not detect the hole.